Fe-mediated HER vs N2RR: Exploring Factors that Contribute to

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Fe-mediated HER vs N2RR: Exploring Factors that Contribute to Selectivity in P3EFe(N2¬) (E = B, Si, C) Catalyst Model Systems Benjamin D. Matson, and Jonas C. Peters ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03068 • Publication Date (Web): 03 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018

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Fe-mediated HER vs N2RR: Exploring Factors that Contribute to Selectivity in P3EFe(N2) (E = B, Si, C) Catalyst Model Systems Benjamin D. Matson and Jonas C. Peters* Division of Chemistry and Chemical Engineering, California Institute of Technology (Caltech), Pasadena, California 91125, United States Abstract: Mitigation of the hydrogen evolution reaction (HER) is a key challenge in selective small molecule reduction catalysis. This is especially true of catalytic nitrogen (N2) and carbon dioxide (CO2) reduction reactions (N2RR and CO2RR, respectively) using H+/e- currency. Here we explore, via DFT calculations, three iron model systems, P3EFe (E = B, Si, C), known to mediate both N2RR and HER, but with different selectivity depending on the identity of the auxiliary ligand. It is suggested that the respective efficiencies of these systems for N2RR trend with the predicted N–H bonds strengths of two putative hydrazido intermediates of the proposed catalytic cycle, P3EFe(NNH2)+ and P3EFe(NNH2). Further, a mechanism is presented for undesired HER consistent with DFT studies, and previously reported experimental data, for these systems; bimolecular proton-coupled-electrontransfer (PCET) from intermediates with weak N–H bonds is posited as an important source of H2, instead of more traditional scenarios that proceed via metal hydride intermediates and proton transfer/electron transfer (PT/ET) pathways. Wiberg bond indices provide additional insight into key factors related to the degree of stabilization of P3EFe(NNH2) species, factors that trend with overall product selectivity. KEYWORDS: nitrogen fixation, N2RR, HER, hydrogen evolution, PCET, HAT, nitrogenase P3BFe and P3CFe.4d,8 We are naturally interested in understanding the mechanism/s by which catalysis in these respective systems occurs, and in exploring alternative systems that might function similarly. Of interest to the present study is the interplay between efficiency for the N2RR and HER on the P3BFe scaffold and its isostructural congeners P3SiFe and P3CFe. In particular, can we elucidate some of the salient factors that dictate overall product selectivity for NH3 versus H2 in these respective systems?

Introduction The reduction of nitrogen (N2) to ammonia (NH3) by nitrogenase enzymes (the nitrogen reduction reaction: N2RR) has garnered substantial interest in the synthetic inorganic community for several decades. 1 In particular, the structural characterization of the FeMo-cofactor active site of biological nitrogen fixation, 2 and mechanistic uncertainties associated with this process, 3 have motivated studies of synthetic (primarily Mo and Fe) model systems that mediate N2RR in the presence of proton and electron equivalents in organic solvent.4-6 The mechanisms of these systems are at various stages of understanding. Experimental4, 5 , 6 and theoretical (predominantly Mo) 7 studies have been undertaken to provide insight. Single-site iron model complexes, such as P3BFe(N2)− and P3BFe+ (Figure 1), catalyze N2RR under a variety of conditions and driving forces, with reported selectivities (to date) for NH3 generation as high as 72% based on reductant consumed.4e In addition, conditions have been reported under which P3CFe(N2)− and P3SiFe(N2)− also catalyze N2RR to varying degrees, with the P3SiFesystem being far more efficient at the hydrogen evolution reaction (HER) than N2RR compared to

Figure 1. Schematic depiction of N2RR/HER iron catalysts studied herein to explore key factors dictating product selectivity. this

1

Herein we use DFT calculations to explore question. We examine the comparative

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feasibility of HER via proton-coupled electron 9 transfer (PCET) from several putative Fe(NxHy) early intermediates, using electronic structure calculations coupled with predicted N–H bond strengths, thermodynamic driving forces, and electron-transfer (ET) kinetics as mechanistic probes. Acknowledging the likelihood that numerous and potentially competing factors may be at play, the formation, electronic structure, and reactivity of a key common intermediate, Fe(NNH2)+, is highlighted to be an important factor in the divergent selectivity profile of P3BFe (and P3CFe) relative to the P3SiFe system.

Reduction kinetics were calculated using the standard Marcus equation relating activation barrier with driving force and total reorganization energy (λtot = λis + λos). 18 The inner-sphere reorganization energy for electron transfer (λis,ET) was estimated assuming non-adiabatic behavior and by calculating the difference between the single point energies of the relevant species in its ground state and the corresponding single point energy of this ground state in the oxidized or reduced geometry (Eq. 1).

Computational Methods

The outer-sphere reorganization energy was calculated by assuming a barrier of 1.0 kcal/mol for the reduction of P3BFe(NNH2)+ followed by calculation of λtot using this barrier and λis, as calculated by Eq 1. A continuum solvation model was used to confirm that λos ≈ λis (See SI for full Reduction barriers for description).18 P3C/SiFe(NNH2)+ were subsequently calculated relative to P3BFe(NNH2)+.

λis,ET = [E(Feoxox) – E(Feoxred)] + [E(Feredred) – E(Feredox)]

All calculations were performed using dispersion corrected density functional theory (DFTD3) using Grimmes dispersion correction. 10 All calculations were done using the full P3EFe scaffold with the TPSS functional11 and a def2-TZVP basis set on transition metals and a def2-SVP basis set on all other atoms.12 All stationary point geometries were optimized using NWChem 6.313 or Orca 3.0.3.14 To ensure consistency in grid size, all reported single point and thermodynamic energies were performed using Orca 3.0.3. Frequency calculations were used to confirm the presence of true minima and to obtain gas phase free-energy values at 195 K (Ggas). Solvation corrections were performed using the COSMO-SMD continuum model. 15 The solvation free energy was approximated using gas phase and solvated single point energies (DGsolv ≈ Esoln - Egas). Finally, the free-energy of the solvated species at 195 K was calculated using the gas-phase freeenergy and the solvation free-energy (Gsoln,195K = Ggas,195k + DGsolv).16 The accuracy of the described computational methodology was measured by comparison to several experimental benchmarks of interest. In addition to ensuring good agreement between computed and crystallographically determined structural data, experimentally determined bond dissociation enthalpies (BDFEN-H) of the compounds P3SiFe(CNH)+, P3SiFe(CNH), P3SiFe(CNMeH)+, P3SiFe(CNMeH) and P3SiFe(NNMeH)+ could be faithfully reproduced within ±2 kcal/mol (See SI for full description).4h As a further point of calibration, the calculated singlet-triplet energy gap and the redox potentials of P3BFe(NNMe2) and Si + P3 Fe(NNMe2) are in good agreement with the experimentally determined values (within ±1.5 kcal/mol, and ±3 kcal/mol (±130 mV vs Fc+/0), respectively ; see SI).4g,h,17

(1)

Results and Discussion To set the stage for the present study, previously reported catalytic N2-to-NH3 conversion studies by P3EFe (E = B, C, and Si) under an atmosphere of N2 at −78 °C in Et2O, using KC8 and [(Et2O)2H][BArF4] (HBArF4, BArF4 = tetrakis-(3,5bis(trifluoromethyl)phenyl)borate) as the reductant and acid source,4a,d established P3BFe as the most efficient catalyst for N2RR; the highest reported efficiency for this system (under these conditions) was 45 ± 3% (48 equiv acid; 58 equiv reductant). For comparison, the P3SiFe system provided a conversion efficiency of only 5 ± 3%. The P3CFe catalyst system was reasonably active at 36 ± 6% (note: ~25% lower substrate loading was used for this P3CFe value4d). Measurement of HER activity established P3SiFe(N2)− (88% per added acid equiv) as a significantly more efficient HER catalyst than P3BFe(N2)− (40% per added acid equiv) under analogous conditions.4d N2RR catalysis by P3EFe (E = B, Si) has also been studied in the presence of milder reagents (e.g., Cp*2Co and [H2NPh2][OTf] or [H3NPh][OTf]); under these conditions only the P3BFe system is catalytically active.

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products.4b,d We posit that reactive P3EFe(NxHy) intermediates instead undergo net bimolecular HAT reactions to liberate H2 via NxHy-ligand-mediated steps (vida infra). While iron hydrides (Fe–H) can tie up the population of active catalyst, in our view they are unlikely to be intermediates of the dominant HER pathway. To speak to this hypothesis computationally, we focus on one acid source, HBArF4, as it has been the subject of the most extensive comparative study.4d The solid-state empirical formula of HBArF4 reveals the presence of two ethers per HBArF4 20 ([(Et2O)2H][BArF4]) To determine the preferred solution-state structure of this acid, optimizations were performed in which a Et2OH+ species was provided with 0, 1 or 2 explicit Et2O molecules with which to hydrogen bond. We found that [(Et2O)2H]+ speciation was lowest in free-energy, with [(Et2O)3H]+ and [Et2OH]+ higher in energy by +7.0 and +8.2 kcal/mol, respectively (see SI).

Figure 2. (top) Previous experimental work showing the formation of P3EFe(NNH2)+ (E = B or Si) via protonation with excess acid.4f,g (bottom) Calculated free energy changes (in kcal/mol; 195 K) for the formation of P3EFe(NNH2)+ via P3EFe(NNH) (E = B or Si).

The structure of HBArF4 is particularly crucial for Fe protonation, as a pre-equilibrium formation of the [Et2OH]+ appears to be required, as evidenced by relaxed surface scans. The need for dissociation of Et2O prior to Fe protonation provides a lower bound on the barrier of +8.2 kcal/mol. The requirement of [Et2OH]+ as the active acid, as opposed to [(Et2O)2H]+, is presumably steric in origin and may speak, in part, to the importance of bulky isopropyl-phosphino substituents in these catalysts. Our lab recently reported that a structurally related P3SiOs(N2)- complex is an active catalyst for N2RR. 21 In contrast to the P3EFe(N2)− catalysts, stoichiometric HBArF4 addition can protonate at the metal, generating Os-H species that are not catalytically active for N2RR. Steric access to the larger Os center is presumably less restricted than it is for Fe.

Previous studies of the P3BFe and P3SiFe systems have also explored the generation and characterization of early stage intermediates of the N2RR catalysis.4e,f,g Most salient, low temperature protonation of P3EFe(N2)− (E = B, Si) with excess HBArF4 affords the doubly protonated P3EFe(NNH2)+ species (Figure 2).4f,g As expected, corresponding DFT calculations (this work) are consistent with thermodynamically favored formation of P3EFe(NNH) via proton transfer (Figure 2); another favorable proton transfer forms P3EFe(NNH2)+ . DFT Support for Slow Fe Protonation and Fast Fe-NxHy Formation

The steric profile of the Fe(N2) unit suggests that functionalization of the β–N should not be subject to the same pre-equilibrium. This is consistent with relaxed surface scans, which show that the N2 unit can be protonated in a concerted, low energy step in which an Et2O molecule is favorably displaced by the nucleophilic β N-atom. Subsequent proton transfers yield Fe(NNH) with a low overall kinetic barrier (0.5–1.0 kcal/mol; see SI).

Although metal hydride (M–H) species are most typically invoked as intermediates of transition-metal catalyzed HER, 19 we do not think Fe–H species are the primary players in H2 formation by the present systems. Several experimental observations are consistent with this idea. Foremost among them is that low temperature addition of stoichiometric acid (e.g., HBArF4) to any of the anions, P3EFe(N2)−, causes overall oxidation to their corresponding neutral products, P3EFe(N2), along with release of 0.5 equiv H2.4a,d This is noteworthy because for E = Si or C the diamagnetic hydride products, P3EFe(N2)(H), are very stable species and are formed during catalysis as end

Fe–H formation is thermodynamically favored for all three scaffolds. We therefore presume that the dominant source of HER for these systems is not via Fe–H formation, but that hydride species are formed over the course of catalysis as thermodynamic products. We presume that both

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HER and N2RR, under the conditions explored in this work, are operating under kinetic control. In subsequent results and discussion, thermodynamics are assumed to be relevant within the context of kinetic parameters.

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downstream intermediates, contrasting the high degree of solution instability of earlier intermediates. Of particular interest herein is that the BDFEN–H values for the P3SiFe(NNH2)n+ (n = 0, 1) system are lower than those for P3B/C, for a given overall charge. As discussed later, these different BDFEN–H values are rooted in the different valence electron counts, and hence electronic structures, of the respective P3EFe-systems. For additional context, it is useful to consider reported BDFEN–H data for a related P3SiFe(CN)-system. The relevant P3SiFe(CNH) species, isoelectronic with P3BFe(NNH), is calculated to have a weak BDFEN–H of 43.5 kcal/mol, in close agreement to that of 41.4 kcal/mol Accordingly, determined experimentally.4h Si P3 Fe(CNH) loses 0.5 equiv H2 rapidly in solution to afford P3SiFe(CN). In contrast, its oxidized cation, P3SiFe(CNH)+, has a much higher BDFEN–H (61.8 kcal/mol (calc); 61.9 kcal/mol (exp)); this species is stable to H2 loss in solution and can be isolated and structurally characterized. Considering these collected data and observations, and additional data discussed below, we presume that the earliest N2RR intermediates in P3EFe-systems are very important for determining N2RR versus HER selectivity; they engage in bimolecular H2-evolving reactions that compete with productive N2RR. We next consider aspects of the H–H bond-forming steps in these early P3EFe(NxHy) intermediates in more detail.

In addition to restricting our analysis to a single acid, HBArF4, we focus on KC8 as a reductant for several reasons. Most salient is that KC8 is the only reductant that has been shown to produce catalytic yields of NH3 for all scaffolds considered. This observation is attributed to the requirement of Fe(N2)− formation during catalysis. While P3BFe(N2)− can be formed with weaker reductants, namely Cp*2Co, the more reducing P3Si/CFe(N2)− is believed to be inaccessible under these conditions. Additionally, it has been noted that, when using KC8 and HBArF4, HER and N2RR proceed with similar initial rates on P3SiFe and P3BFe scaffolds,4d possibly due to Fe(N2) reduction being a common rate limiting step. Further the initial rates of N2RR are similar between the scaffolds, which makes this reductant/acid combination well suited for a comparative study. Despite the need to restrict the scope of this study to a specific catalysis cocktail, many of the conclusions should extend to other conditions reported for N2RR catalysis using P3EFe (and related) complexes. In particular, the BDFEN-H values reported herein are acid and reductant independent and hence provide insight into the anticipated stability and reactivity profiles of key early intermediates of N2RR.

H

iPr

2P

Calculation of BDFEN-H Values for Fe–NxHy Intermediates

H

N N i Fe P Pr2 PiPr2 E

iPr

P3EFe(NNH) E BDFEN-H 31.2 B 23.0 Si 17.3 C

Early stage intermediates of the type Fe(NNH) and Fe(NNH2) are expected to be highly reactive;4e,h thermochemical calculations reveal the presence of extremely weak N–H bonds in these systems, as shown by their calculated bond dissociation enthalpies (BDFEN–H; Figure 3). In particular, as yet unobservable P3EFe(NNH) intermediates are predicted to have extremely weak N–H bonds (< 40 kcal/mol), and should therefore be subject to rapid bimolecular loss of H2 and generation of P3EFe(N2). By contrast, the BDFEN-H values of candidate P3EFe(NxHy) intermediates that are further downstream (e.g., Fe(N2H4), Fe(NH), Fe(NH2)) are predicted to be significantly larger (Figure 3). This notion is consistent with the solution stability of characterized examples of such

N

+

H

N i Fe P Pr2 PiPr2 E

2P

H N H iPr

P3EFe(NNH2)+ E BDFEN-H 49.7 B 39.1 Si 40.6 C

H N H H H N iPr iPr P 2 2 P Fe PiPr2 E

P3EFe(NH2NH2)

N i Fe P Pr2 PiPr2 E

iPr 2P

H N

+/0

+/0

i Fe P Pr2 PiPr2 E

P3EFe(NH)+/0

N i Fe P Pr2 PiPr2 E

P3EFe(NNH2) E BDFEN-H Si 22.9 C 34.5

P3EFe(NNH2) E BDFEN-H 38.2 B

+

iPr 2P

2P

H N H

NH2 iPr

2P

i Fe P Pr2 PiPr2 E

P3EFe(NH)+/0

(E = B or Si)

(E = B or Si)

(E = B or Si)

BDFEN-H: 58-60

BDFEN-H: 54-65

BDFEN-H: 70-80

Figure 3. BDFEN–H values (in kcal/mol) for selected P3EFe(NxHy) species.22 species are plausible P3EFe(NNH) candidates to consider with respect to selectivity

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accomplished by the strong reductant KC8, P3C/SiFe(NNH2)+ species are predicted to be significantly longer lived than the P3BFe(NNH2)+ congener. To roughly quantify the differences in reduction rate between P3EFe(NNH2)+ species, and hence get a sense of their relative expected lifetimes, we turned to transition state theory. By assuming a pre-exponential factor invariant across both scaffolds, reduction rates for P3SiFe(NNH2)+ and P3CFe(NNH2)+, normalized to P3BFe(NNH2)+ (krel), were calculated (krel = 2x10−4 and 2x10−5, respectively). Accordingly, we expect P3SiFe(NNH2)+ and P3CFe(NNH2)+ to be ~ 104 times longer-lived, respectively, than P3BFe(NNH2)+, with respect to one-electron reduction. We conclude that facile reduction of P3BFe(NNH2)+ to P3BFe(NNH2), relative to that for P3SiFe(NNH2)+ and P3CFe(NNH2)+, is one important factor in determining its comparative efficiency for N2RR. As further elaborated below, long-lived P3EFe(NNH2)+ intermediates can, via bimolecular PCET pathways, instead lead to unproductive HER. This HER activity, however, is dependent on both a long-lived P3EFe(NNH2)+ intermediate, and the presence of a highly reactive PCET reagent, such as a P3EFe(NNH2) species. We have previously postulated that P3EFe(NNH2) formation is required for the release of the first equivalent of NH3 and thus suggest that this species may be a crucial intermediate in both HER and N2RR.4fg,17

since bimolecular H2-evolving reactions can presumably result from their extremely weak N−H bonds (Figure 3; 31-17 kcal/mol). P3SiFe(NNH), with a BDFEN–H estimated to be 8.2 kcal/mol lower than for P3BFe(NNH), might be reasonably expected to liberate H2 more readily, thereby attenuating its N2RR efficiency. However, the BDFEN–H for P3CFe(NNH) is calculated to be even lower (17.3 kcal/mol) than for P3SiFe(NNH) (23 kcal/mol), despite the fact that P3CFe(N2)− is appreciably more efficient for N2RR. Hence, a trend is not evident on the basis of the Fe(NNH) intermediates, at least as related to their relative BDFEN–H values. Fe(NNH) intermediates are readily protonated to form Fe(NNH2)+ species in solution at low temperature (Figure 2). This likewise suggests that Fe(NNH) intermediates are unlikely to be primarily responsible for HER under catalytic conditions when a large excess of acid is present.4f,g P3EFe(NNH2) BDFEN–H values provide a more tractable trend: the respective calculated values are 38.2 kcal/mol for P3BFe, 34.4 kcal/mol for P3CFe, and 22.9 kcal/mol for P3SiFe; the P3EFe–NNH2 species that exhibits the most efficient N2RR activity exhibits the strongest N-H bond, and the least efficient exhibits the weakest (Figure 3). Calculated Reduction Kinetics of P3EFe(NNH2)+ To gain further insight into the respective role P3EFe(NNH2)+/0 (E = B, Si, C) species might play in dictating product selectivity, P3EFe(NNH2)+ reduction kinetics were derived using the standard Marcus equation relating the driving force and total reorganization energy with the ET activation barrier.18 Comparison of the optimized Fe(NNH2) and Fe(NNH2)+ redox pairs reveals significant differences in their respective reduction potentials and inner-sphere reorganization energies (λis,ET). The P3BFe(NNH2)+ species is predicted to have a considerably more positive reduction potential (−1.2 V vs Fc/Fc+) than P3SiFe(NNH2)+ (−1.9 V; Table 1), resulting from their different valence electronic counts and electronic structures (see below). Given their dramatic difference in reduction potentials, the barrier for reduction (G*) is expected to sharply increase in moving from B to Si. Relative reduction barrier calculations, assuming G* = 1.0 kcal/mol for the reduction of P3BFe(NNH2)+, predict activation barriers that are 4–5 times higher in energy for the reduction of P3C/SiFe(NNH2)+ versus P3BFe(NNH2)+ (Table 1). While the reduction of all three species should be more than readily

Table 1. Calculated thermodynamic and kinetic parameters for P3EFe(NNH2)+ à P3EFe(NNH2)a P3EFe(NNH2)+ + e- à P3EFe(NNH2) Eo (vs Fc+/0)

λis,ET

G*relb

krelb

E=B −1.2 V 23 1.0 1 E = Si −1.9 V 30 4.4 2x10−4 E=C −2.0 V 30 5.2 2x10−5 a Energies are in kcal/mol, unless noted otherwise. b G*rel values were calculated assuming a P3BFe(NNH2)+ reduction barrier of 1.0 kcal/mol. krel ≡ exp[(G*B−G*E)/kbT] where T = 195 K.

Calculated PCET Reactions The differences in N–H bond strengths and relative rates of P3EFe(NNH2)+ reduction, with corresponding implications for product selectivity, are further highlighted by calculating the thermodynamic and kinetic parameters for several PCET reactions of interest (Figure 4ABC). In

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proceed with far more driving force than the P3BFe system. Favorable driving forces are also predicted for all three systems in self reactions of P3EFe(NNH2) to produce H2 and P3EFe(NNH), but again the P3C/SiFe systems proceed with far more driving force (Figure 4B). While the bimolecular reaction of P3EFe(NNH2) with itself is therefore a presumed source of H2 for each system, in sum the P3C/SiFe systems are more likely, under each of the considered bimolecular reactions, to liberate H2, in accord with their efficiency for HER versus N2RR relative to the P3BFe system. Given that the reduction of P3C/SiFe(NNH2)+ is predicted to be comparatively slow, one might expect such a species to build-up as an intermediate. This possibility warrants future experimental studies aimed at in situ detection. At the present stage, we can suggest that a high (relative) concentration of P3C/SiFe(NNH2)+, and a high predicted propensity for HER via reaction of this species with either itself or P3C/SiFe(NNH2), leads to unproductive PCET steps that evolve H2 as competitive with downstream N2 reduction steps that lead to N2RR. This is one important factor in determining selectivity.

particular, comparative driving forces were calculated for unproductive bimolecular PCET reactions that generate H2 between P3EFe(NNH2)n+ (n = 0,1; E = B, Si, C) and P3EFe(NNH2). Consistent with the calculated BDFEN–H values (Figure 3), the P3SiFe, and to a lesser extent the P3CFe, system shows a higher propensity to undergo PCET to liberate H2 and the corresponding reduced Fe–NNHy species. This is especially apparent in the reaction between two P3EFe(NNH2)+ species, and in the cross-reaction between an P3EFe(NNH2)+ cation and a neutral P3EFe(NNH2) species. In the former case, two P3EFe(NNH2)+ (E = Si, C) species react in a very favorable step to form 0.5 equiv H2 and P3EFe(NNH)+ (ΔGcalc = −17.5 kcal/mol and −16.5, respectively; Figure 4A). The reaction barrier is expected to be dominated in this case by the work required to bring two cationic species together in solution (~5 kcal/mol; see SI), highlighting the reactive nature of P3C/SiFe(NNH2)+. In contrast, P3BFe(NNH2)+ shows a correspondingly uphill PCET reaction (ΔGcalc = +3.1 kcal/mol) in its self-combination to liberate H2 and P3BFe(NNH)+;23 P3BFe(NNH2)+ is also much more readily reduced to P3BFe(NNH2) (Table 1).

N N iPr P Fe PiPr2 2 PiPr2 E

2 NH3 + 4 H+ + 4 e-

+ e-/H+

N2

Bimolecular HER

NH2

NH

N iPr 2P

N

i Fe Pi Pr2 P Pr2 E

iPr 2P

BDFE: 38 kcal/mol (B) 34 kcal/mol (C) 23 kcal/mol (Si)

i Fe Pi Pr2 P Pr2 E

BDFE: 32 kcal/mol (B) 17 kcal/mol (C) 24 kcal/mol (Si) NH2 + e-

E

All P3 (E = B, C, Si) P3Si and P3C primarily

N iPr 2P

Minor HER Pathway Major HER Pathway

Not correlated with catalyst efficiency

i Fe Pi Pr2 P Pr2 E

+ H+ BDFE: 49 kcal/mol (B) 45 kcal/mol (C) 39 kcal/mol (Si)

Figure 5. Overview of predicted bimolecular HER and N2RR pathways for P3EFe(NNHy) species and pertinent BDFEN–H values.

Figure 4. Calculated free energy changes (ΔGcalc; in kcal/mol; 195 K) for several putative PCET reactions that evolve H2.

Since the P3BFe(NNH2)+ intermediate is predicted to have a lower propensity for H2liberating PCET reactivity, and is also predicted to be reduced much more rapidly, the reaction of two P3BFe(NNH2) molecules is a more probable source of H2 for this scaffold; the efficiency for N2RR on P3BFe should therefore be related to the rate at which

The bimolecular reaction between cationic P3EFe(NNH2)+ with P3EFe(NNH2) to produce H2 and the corresponding P3EFe(NNH)+ and P3EFe(NNH) byproducts is predicted to be favorable for all three systems (Figure 4C). However, the P3C/SiFe systems

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P3BFe(NNH2) can be productively consumed (i.e., protonated to form a P3BFe(NHNH2)+ or P3BFe(NNH3)+). Mechanistic experiments to address these scenarios are ongoing. For example, a recent study has shown that P3BFe(NNH2) can be protonated by strong acid at low temperature to liberate P3BFe(N)+ and NH3, presumably via P3BFe(NNH3)+.17 While the P3CFe scaffold provides a less definitive comparison, the calculated BDFEN–H values and H2-evolving PCET thermodynamics suggest that the dominant source of HER on the P3C/SiFe scaffolds may be the reaction between Fe(NNH2) and Fe(NNH2)+. The highly reducing nature of P3CFe(NNH2)+, as for the P3Si scaffold, suggests it should be comparatively long-lived, and thus more likely to undergo PCET with P3CFe(NNH2). The similarity between P3CFe and P3SiFe in their thermodynamics for the reaction between two Fe(NNH2)+ species (Figure 4A) does not correlate with their disparate %NH3 efficiencies. Substantial differences in their predicted thermodynamics for the reaction between Fe(NNH2) and Fe(NNH2)+ (Figure 4C) are more in line with the observed trend. This type of bimolecular reactivity may be an important source of HER on the P3C/SiFe scaffolds (Figure 5).

the former. Interestingly, the N–H bond indices are essentially invariant across all complexes studied, indicating that differences in BDFEN–H are mostly dependent on the relative bonding through the E– 26 Fe–N–N manifold. The most salient data, reproduced in Figure 6, are the total Wiberg bond indices for Fe−Nα, Fe−Nβ, Fe−E, N−N and N−H. The total Fe−N–N bond order, ∑(Fe−N−N), is also provided, as is the net difference in the DBDFEN-H value, for each pair on moving from Fe(NNH) to Fe(NNH2). As expected, the Fe–E bond order weakens slightly from Fe(NNH) to Fe(NNH2) for E = B, and stays constant for both Si and C. The respective change at Fe-Nα is also informative. For the B system, a significant increase is observed (1.6 to 1.9), reflecting a build-up in pi-bonding in P3BFe(NNH2), akin to low-spin (pseudotetrahedral) iron imides of the type P3BFe(NR). For comparison, a previously characterized P3BFe(NR) species (R = 4-OMe-Ph) is predicted to have an Fe–N bond order of 1.8 (see SI). By contrast, the Fe–Nα index for Si is sharply attenuated (from 1.6 to 1.2), reflecting a corresponding decrease in pi bonding. While this difference must partly reflect a less flexible Fe-Si interaction, it also reflects the electronic structure resulting from an extra electron in the frontier orbitals of the 2E {Fe-Si}7 system relative to 1A {FeB}6. Interestingly, P3BFe(NNH2) is pyramidalized at Nβ whereas Nβ is planar for P3SiFe(NNH2). This observation can again be rationalized by the assignment of a low-spin iron “imide-like” electronic structure to {Fe-B}6 P3BFe(NNH2), but not for {Fe-Si}7 P3SiFe(NNH2), where substantial spin leaks onto the NNH2 subunit (19% on P3SiFe(NNH2)). The C system provides an interesting further comparison, with spin leakage onto the NNH2 unit falling between these two extremes (12% on P3CFe(NNH2)). An increase in the Fe-Nα index occurs from P3CFe(NNH) to P3CFe(NNH2) (1.2 to 1.4), but Nβ is predicted to remain planar. There also appears to be a strong trend between the degree of change in the total Fe−N−N bond order (∑(Fe−N−N)) and the DBDFEN-H; The B and C systems show little change in ∑(Fe−N−N), with a corresponding significant increase in BDFENH from Fe(NNH) to Fe(NNH2) (7.0 and 17.9 kcal/mol, respectively). However, the P3CFe system starts at a much weaker BDFEN-H of 17.3 kcal/mol for P3CFe(NNH) (compared to 31.2 kcal/mol for P3BFe). This observation is consistent with their total ∑(Fe−N−N) values (3.8 for B and 2.9 for C). Thus, the comparative stability of P3CFe(NNH2), with its much higher BDFEN-H relative that in P3CFe(NNH),

Wiberg Bond Indices of P3EFe(NxHy) Species We next examine how each P3E auxiliary, and the corresponding P3EFe(NNHy) valence at iron, confers variability in bonding to, and the electronic structure of, the NNHy ligand, as a means of further considering corresponding reactivity differences of P3EFe(NNHy) species. Wiberg bond indices provide a means to examine how the localized bonding between various 24 atoms, expressed as a bond index, changes as a function of the NNHy reduction state (i.e., NNH to NNH2). We have suggested elsewhere that the relative flexibility of the P3B ligand, owing to a weak and dative Fe®B interaction, may allow for stabilization of Fe−NNHy intermediates where Fe–N pi-bonding is accompanied by pyramidalization at the Fe center, and a corresponding lengthening of the Fe–B distance.4a,17,25 The P3Si ligand is expected to give rise to a more shared, covalent Fe–Si interaction, irrespective of the NNHy reduction state, and the P3C system may be expected to fall in the middle of these extremes.4b Changes in the respective bond indices of these frameworks have been determined between pairs of P3EFe(NNH) and P3EFe(NNH2) species (E = B, C, Si), related by formal addition of an H-atom to

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appears to reflect a higher degree of instability in P3CFe(NNH) (relative to the same comparison for E = B). This idea is further supported by Wiberg bond indices of the P3EFe(N2) species, which show a total bond order of 4.0 across the Fe–N–N unit for all three scaffolds (Figure 6). In sharp contrast, the P3SiFe system has a relatively high ∑(Fe−N−N) value in P3SiFe(NNH), but this value decreases dramatically in P3SiFe(NNH2). There is correspondingly very little change in the DBDFEN-H, reflecting a comparatively very weak N-H bond in P3SiFe(NNH2). The instability of P3SiFe(NNH2), with an electronic structure that places substantial unpaired spin on NNH2 owing to the {Fe-Si}7 configuration, presumably contributes to the cathodically shifted reduction potential predicted for P3SiFe(NNH2)+ relative to P3BFe(NNH2)+, and also its propensity for facile PCET to liberate H2. The P3BFe system is unique within this series in its ability to support a high total Fe−N−N bond order from Fe(NNH) to Fe(NNH2), facilitating its trajectory along productive N2RR. N 0.4

N Fe

Figure 6. Selected total Wiberg bond indices for P3EFe(N2), P3EFe(NNH) and P3EFe(NNH2) species, along with the total Fe−N–N bond order, ∑ (Fe−N−N). DBDFEN-H values are reported in kcal/mol. Conclusion Exploring the chemical basis for N2RR versus HER selectivity for a molecular catalyst is important to future catalyst design. The DFT study described herein suggests that PCET reactions involving P3EFe(NNH2)n+ species likely play an important role in the efficiency of N2-to-NH3 conversion catalysis by P3EFe model systems. These calculations enable predictions qualitatively consistent with previous stoichiometric and catalytic experiments. The comparative stability of P3EFe(NNH2)n+ intermediates, as predicted by calibrated BDFEN–H values and redox potentials, emerges as one of the important factors in determining selectivity for N2RR versus HER in these systems. Corresponding Wiberg bond indices intimate P3B as an especially well-equipped ligand for supporting N2RR at Fe, due to its high degree of flexibility and the valence electron count it confers to Fe in the reduced intermediate P3BFe(NNH2). Our study suggests that increasing the rate at which an P3EFe(NNH2) intermediate is productively consumed so as to avoid bimolecular HER, possibly via rapid PCET reagents, may be a promising route to increasing efficiency for NH3 production. Looking beyond these iron model systems, our study underscores the potential utility of DFTpredicted BDFEN-H determinations towards the rational design of catalysts for N2RR. Intermediates with weak N–H bonds (e.g., M(NNH) and M(NNH2)) are highlighted as important sources of H2 production via bimolecular PCET. Such a scenario is distinct from HER activity via more traditional metal-hydride intermediates.

2.6 1.0 0.5

E (E = B, Si, C)

Σ(Fe-N-N) = 4.0 H 0.8 ea. N H

H 0.8 N 0.4

1.8

N Fe

H・

1.6

Fe

ΔBDFEN-H = +7

0.5

1.2

N

0.2

1.9

0.4

B

B

Σ(Fe-N-N) = 3.8

Σ(Fe-N-N) = 3.5

H 0.8 N 0.4

H 1.4 N

1.8

N Fe

H・

1.6

ΔBDFEN-H = - 1

0.7

Si

Σ(Fe-N-N) = 3.8

N Fe C

Fe Si

H

0.2

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Complete computational details, molecular geometries from the computations, additional computations and analysis (PDF)

0.7

H

0.8 ea.

1.4 N

1.5

1.2

H・

0.5

ΔBDFEN-H = +17

Σ(Fe-N-N) = 2.9

N

0.8 ea.

Σ(Fe-N-N) = 2.9

H 0.8 N 0.2

1.2

1.4

H

N

Fe C

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0.1 0.5

AUTHOR INFORMATION Corresponding Author *[email protected]

Σ(Fe-N-N) = 2.9

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Funding Sources No competing financial interests have been declared.

Chem. 2015, 54, 9256–9262. (b) Kuriyama, S.; Arashiba, K.; Tanaka, H.; Y. Matsuo, Y.; Nakajima, K.; Yoshizawa, K.; Nishibayashi, Y. Angew. Chem. Int. Ed. 2016, 55, 14291–14295. 7 (a) Reiher, M.; Le Guennic, B.; Kirchner, B. Inorg. Chem. 2005, 44, 9640–9642. (c) Studt, F.; Tuczek, F. Angew. Chem. Int. Ed. 2005, 44, 5639–5642. (d) Studt, F.; Tuczek, F. J. Comput. Chem. 2006, 27, 1278–1291. (e) Ujjal Gogoi, U.; Kanti Guha, A.; Phukan, A. K. Chem. Eur. J. 2013, 19, 11077–11089. 8 Side-by-side comparisons of catalytic N2RR by P3EFe (E = B, C, and Si) using potassium graphite (KC8) and [(Et2O)2H][BArF4] (HBArF4, BArF4 = tetrakis-(3,5bis(trifluoromethyl)phenyl)borate) have revealed P3BFe to be the most efficient catalyst, with reported efficiencies up to 37%. N2RR catalysis by P3CFe or P3SiFe is less efficient under these conditions, with reported efficiencies of 33% and 4%, respectively. Studies focused on HER catalysis have shown P3SiFe(N2)- (88% per H+) to be significantly more efficient than P3BFe(N2)- (40% per H+) under analogous conditions. See 4d. 9 In this study, we do not distinguish between the terms H-atom transfer (HAT) vs PCET. For consistency, we refer to PCET throughout. For discussion of terminology see: (a) Warren, J. J.; Tronic, T. A.; Mayer, J. M.. Chem. Rev. 2010, 110, 6961–7001. (b) Hammes-Schiffer, S. J. Am. Chem. Soc. 2015, 137, 8860−8871. 10 Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. J. Chem. Phys. 2010, 132, 154104. 11 Tao, J. M.; Perdew, J. P.; Staroverov, V. N.; Scuseria, G. E. Phys. Rev. Lett. 2003, 91, 146401. 12 Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7, 3297–3305. 13 Valiev, M.; Bylaska, E. J.; Govind, N.; Kowalski, K.; Straatsma, T. P.; Van Dam, H. J. J.; Wang, D.; Nieplocha, J.; Apra, E.; Windus, T. L.; de Jong, W. A. Comput. Phys. Commun. 2010, 181, 1477–1489. 14 Neese, F. Wiley Interdiscip. Rev. Comput. Mol. Sci. 2012, 2, 73-78. 15 (a) Klamt, A.; Schüürmann, G. J. Chem. Soc. Perkin Trans. 2. 1993, 2, 799–805. (b)Marten, B.; Kim, K.; Cortis, C.; Friesner, R. A.; Murphy, R. B.; Ringnalda, M. N.; Sitkoff, D.; Honig, B. J. Phys. Chem. 1996, 100, 11775–11788. 16 (a) Ribeiro, R. F.; Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B 2011, 115, 14556–14562. (b) Wang, T.; Brudvig, G.; Batista, V. S. J. Chem. Theory Comput. 2010, 6, 755–760. (c) Marten, B.; Kim, K.; Cortis, C.; Friesner, R. A.; Murphy, R. B.; Ringnalda, M. N.; Sitkoff, D.; Honig, B. J. Phys. Chem. 1996, 100, 11775–11788. 17 Thompson, N.B.; Green, M. T.; Peters, J.C. J. Am. Chem. Soc., 2017, 139, 16105-16108. 18 Marcus, R. A. J. Chem. Phys. 1956, 24, 966–978. 19 (a) Dempsey, J. L.; Brunschwig, B. S.; Winkler, J. R.; Gray, H. B. Acc. Chem. Res. 2009, 42, 1995–2004. (b) Lewandowska-Andralojc, A.; Baine, T.; Zhao, X.; Muckerman, J. T.; Fujita, E.; Polyansky, D. E. Inorg.

ACKNOWLEDGMENT This work was supported by the NIH (GM 070757) and the Gordon and Betty Moore Foundation, and the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number ACI1053575. B.D.M. acknowledges the support of the NSF for a Graduate Fellowship (GRFP). We thank Matthew Chalkley for insightful input. REFERENCE 1

(a) Chatt, J.; Dilworth, J. R.; Richards, R. L. Chem. Rev. 1978, 78, 589-625. (b) Schrock, R. R. Acc. Chem. Res. 2005, 38, 955-962. (c) Nishibayashi, Y. Inorg. Chem. 2015, 54, 9234−9247. (d) van der Ham, C. J. M.; Koper, M. T. M.; Hetterscheid, D. G. H. Chem. Soc. Rev. 2014, 43, 5183−5191. (e) Shaver, M. P.; Fryzuk, M. D. Adv. Synth. Catal. 2003, 345, 1061−1076. (f) MacLeod, K. C.; Holland, P. L. Nat. Chem. 2013, 5, 559−565. 2 (a) Howard, J. B.; Rees, D. C. Proc. Nat. Acad. Sci. 2006, 103, 17088-17093. (b) Howard, J. B.; Rees, D. C. Chem. Rev. 1996, 96, 2965−2982. 3 Hoffman, B. M.; Lukoyanov, D.; Yang, Z.-Y.; Dean, D. R.; Seefeldt, L. C. Chem. Rev. 2014, 114, 4041−4062. 4 For pertinent Fe systems: (a) Anderson, J. S.; Rittle, J.; Peters, J. C. Nature 2013, 501, 84−87. (b) Creutz, S. E.; Peters, J. C. J. Am. Chem. Soc. 2014, 136, 1105−1115. (c) Ung, G.; Peters, J. C. Angew. Chem., Int. Ed. 2015, 54, 532−535. (d) Del Castillo, T. J.; Thompson, N. B.; Peters, J. C. J. Am. Chem. Soc. 2016, 138, 5341–5350. (e) Chalkley, M. J.; Del Castillo, T. J.; Matson, B. D.; Roddy, J. P.; Peters, J. C. ACS Central Science 2017, 3, 217-223. (f) Anderson, J. S.; Cutsail, G. E.; Rittle, J.; Connor, B. A.; Gunderson, W. A.; Zhang, L.; Hoffman, B. M.; Peters, J. C. J. Am. Chem. Soc. 2015, 137, 7803–7809. (g) Rittle, J.; Peters, J. C. J. Am. Chem. Soc. 2016, 138, 4243–4248. (h) Rittle, J.; Peters, J. C. J. Am. Chem. Soc. 2017, 139, 3161–3170. 5 For Mo systems: (a) Kuriyama, S.; Arashiba, K.; Nakajima, K.; Matsuo, Y.; Tanaka, H.; Ishii, K.; Yoshizawa, K.; Nishibayashi, Y. Nat. Commun. 2016, 7, 12181. (b) Yandulov, D. V.; Schrock, R. R. Science 2003, 301, 76−78. (c) Arashiba, K.; Miyake, Y.; Nishibayashi, Y. Nat. Chem. 2011, 3, 120−125. (d) Kuriyama, S.; Arashiba, K.; Nakajima, K.; Tanaka, H.; Kamaru, N.; Yoshizawa, K.; Nishibayashi, Y. J. Am. Chem. Soc. 2014, 136, 9719−9731. (e) Arashiba, K.; Kinoshita, E.; Kuriyama, S.; Eizawa, A.; Nakajima, K.; Tanaka, H.; Yoshizawa, K.; Nishibayashi, Y. J. Am. Chem. Soc. 2015, 137, 5666−5669. 6 For Co systems: (a) Del Castillo, T. J.; Thompson, N. B.; Suess, D. L. M.; Ung, G.; Peters, J. C. Inorg.

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Chem. 2015, 54, 4310–4321. (c) Yang, J. Y.; Smith, S. E.; Liu, T.; Dougherty, W. G.; Hoffert, W. A.; Kassel, W. S.; DuBois, M. R.; DuBois, D. L.; Bullock, R. M. J. Am. Chem. Soc. 2013, 135, 9700–9712. 20 Brookhart, M.; Grant, B.; Volpe Jr., A. F. Organometallics 1992, 11, 3920–3922. 21 Fajardo, J.; Peters, J. C. J. Am. Chem. Soc. 2017, 139, 16105–16108. 22 We have previously reported bond dissociation enthalpies (BDEN-H) for P3BFe(NNH) and P3BFe(NNH2) (see ref 4e). Here we reported BDFEN-H values as they have more theoretical justification in the absence of experimental knowledge of the entropy change associated with H· loss.

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While this discussion may seem at odds with the enhanced stability of P3SiFe(NNH2)+ relative to P3BFe(NNH2)+, other factors are presumably responsible in solution, such as the more facile reduction of P3BFe(NNH2)+ relative to P3SiFe(NNH2)+. 24 Wiberg, K. B. Tetrahedron, 1968, 24, 1083-1096. 25 Moret, M-.E.; Peters, J. C. J. Am. Chem. Soc., 2011, 133, 18118-18121. 26 Similarly, the C2H5 radical is predicted to have a very low BDFEC-H (34 kcal/mol) when compared to C2H4 (100 kcal/mol), but the Wiberg bond indices for their respective C–H bonds do not change appreciably (See SI).

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TOC Graphic H2

NXHy-mediated PCET Pathways Can Short-Circuit N2RR via HER

H+/e-

NHy N Unproductive HER

iPr

2P

Fe E

PiPr2 PiPr2

Productive NProductive 2RR N2RR

E = B, Si, C

H+/e-

N2

NH3

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